1. Field of the Invention
The present invention relates to a fiber optic interferometric position sensor and measurement method thereof, and more particularly, to a fiber optic interferometric position sensor and measurement method thereof suitable for determining the moving direction of a measurement object in an environment of high electric or magnetic field strengths.
2. Description of Related Art
At present, the industry primarily adopts a capacitor sensor to serve as a sub-nanometer scale position sensor. However, the capacitor sensor has some drawbacks, as immediately described below, which limit its application scope. At first, if a high electric or magnetic field strength is applied to the neighborhood of the probe of the capacitor sensor during measurement, the reading obtained by the capacitor sensor will be affected. As a result, an error occurs in this measurement. Next, in order to apply the principle of the capacitor to detect the displacement and the position of a measurement object, it is necessary to have an electrode plate pivotally locked on the measurement object prior to the measurement. Due to the huge size of the electrode plate, the displacement of the measurement object is disadvantageously affected, and also, the installation of the electrode plate over the surface of the measurement object becomes complex. As such, the capacitor sensor is generally not suitable for being applied to a sub-nanometer scale position sensing environment where the measurement object is usually smaller and less weight, where the space for installation of the electrode plate is limited or where high noise signals caused by electromagnetic radiation exist.
A fiber optic interferometer configured as shown in FIG. 1a can also serve as a sensor for sub-nanometer scale position sensing measurement. After a laser beam is transmitted from a light source 11 of a fiber optic interferometer 1 to a fiber optic coupler 12, the laser beam is directed into a sensing fiber 13. When the laser beam is incident on a fiber termination 14, a part of the laser beam is reflected back into the sensing fiber 13 and another part of the laser beam passes through the fiber termination 14 and reaches the surface 16 of a measurement object 15. Then, the laser beam incident on the surface 16 of the measurement object 15 is reflected from the surface 16 and travels back to the fiber termination 14, in which the reflected laser beam partially passes through the fiber termination 14 so as to be transmitted into the sensing fiber 13. This partially passed laser beam and the laser beam previously reflected back into the sensing fiber 13 directly from the fiber termination 14 generate an interference effect, resulting in an interference beam. The interference beam is transmitted into a photodetector 17 via the fiber optic coupler 12, and a change in the interference pattern is detected and recorded by the photodetector 17. Thus, the fiber optic interferometer requires only a small reflecting surface on the measurement object to proceed with the measurement, and has a more broad application scope than the aforesaid capacitor sensor. Because there is no electronic element near the end of the measurement object, the value obtained as a result of the measurement will not be sensitive to the electromagnetic radiation of the measurement object.
The operating principle of the aforesaid fiber optic interferometer will be described below.
Turning to FIG. 1b, if the initial intensity of the laser beam transmitted into the sensing fiber 13 from the fiber optic coupler 12 is I0 and the reflective index of the fiber termination 14 is R1, the intensity I1 of the laser beam which is reflected from the fiber termination 14 and travels back to the sensing fiber 13 will be R1I0 while the intensity of the laser beam which passes through the fiber termination 14 will be (1−R1)I0. In addition, if the reflective index of the surface 16 of the measurement object 15 is R2, the intensity I2 of the laser beam which is reflected from surface 16 and again transmitted into the sensing fiber 13 will be (1−R1)2R2I0.
Finally, if γ is the coherence factor of the laser beam, the interference signal detected by the photodetector 17 can be expressed as:I=R1I0+(1−R1)2I0R2+2γ(1−R1)I0√{square root over (R1R2)} cos φ  (Equation 1)where φ is the phase difference between the two light beams I1 and I2, having the following relationship:
  ϕ  =      2    ⁢          d      ·                        2          ⁢          π                λ            ·              n        g            where λ is the wavelength of the laser beam, and ng is the refractive index of the medium outside the fiber.
In addition, the relationship among γ, the coherence length Lc of the laser beam and a gap d is given by:
                    γ        =                              sinc            ⁡                          (                              2                ⁢                                  d                  /                                      L                    c                                                              )                                =                                    sin              ⁡                              (                                  2                  ⁢                  d                  ⁢                                                                          ⁢                                      π                    /                                          L                      c                                                                      )                                                    2              ⁢              d              ⁢                                                          ⁢                              π                /                                  L                  c                                                                                        (                  Equation          ⁢                                          ⁢          2                )            
FIG. 2 is a pattern of an interference signal as described in Equation (1). When the medium outside the sensing fiber 13 is air (namely, the measurement is made in the atmosphere), the period of the interference signal as shown in FIG. 2 is about one-half wavelength of the laser beam.
In this light, because the fiber optic interferometer adopts two measuring beams having different optical path lengths to cause an interference phenomenon for measuring the displacement of the measurement object, the value obtained as a result of such a measurement will not be so sensitive to the electromagnetic radiation, as compared with the value measured by the capacitor sensor, when there is a high electric or magnetic field strength at the end of the measurement object.
Though the aforesaid fiber optic interferometer can measure the amount of a displacement of the measurement object, it cannot detect the moving direction of the measurement object for only one sensing fiber is used. In addition, the resolution of the fiber optic interferometer having a single sensing fiber is one-half wavelength of the laser beam at the most. In other words, a displacement less than one-half wavelength of the laser beam (about hundreds nanometers) cannot be measured by the fiber optic interferometer having a single sensing fiber. Hence, the aforesaid fiber optic interferometer equipped with a single sensing fiber has an extremely narrow application scope, and cannot serve as a position sensor in an application to a device having a tiny displacement, such as a piezoelectric displacement unit, a nanometer controlling unit or a micro-gage.
It is therefore a dire need for the industry to provide a fiber optic interferometric position sensor having high resolution where a reading therefrom will not be sensitive to a high electric or magnetic field strength of the surrounding environment so as to rapidly and precisely measure the displacement and the position of a measurement object.